Waveguide switch having a diode mounted in the side arm of a three port junction



July 13, 1965 D. L. REBSCH 3,195,074

WAVEGUIDE SWITCH HAVING A DIODE MOUNTED IN THE SIDE ARM OF A THREE PURT JUNCTION Filed Jan. 2. 1962 L0 A CENTIMETERS Fig. 4

United States Patent WAVEGUIDE SWITCH HAVING A DIQDE MOUNTED EN THE SIDE ARM 6? A THREE PORT .IUNCTIGN Donald L. Rehseh, Glen Burnie, Md, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa a corporation of Pennsylvania Filed Jan. 2, 1962, Ser. No. 163,696 Claims. (Q1. 3337) This invention relates to apparatus for controlling microwave power, and more particularly to a new and improved microwave switch which has low insertion loss and fast response time.

In many microwave applications, the need arises for a fast acting waveguide switch to serve as either an onoff device or an RF modulator. The conventional mechanical switch, either rotor or vane type, switches in milliseconds and also has the disadvantage of large size when physical volume is considered. The electronic switch of a ferrite type is smaller in size and has switching time measured in microseconds; however, large peak powers are required to drive the solenoid and the problem of holding a large coil current for the duration of a pulse with fast rise time and fall times introduces complexities. It has also been discovered that a crystal diode can be used as the switching element. Because of the small time constant of the crystal diode and its low impedance, there has evolved an electronic switch capable of switching in a fraction of a microsecond and requiring low driving power. In addition, the crystal switch may also serve as an RF modulator or variable attenuator.

Microwave crystal diode waveguide switches have been shown and described in previous publications, for example: Microwave Semiconductor Switching Techniques, R. V. Garver et al., IRE Transactions on Microwace Theory and Techniques, volume MTT-6, October 1958, pages 378 to 383; and Microwave Semiconductor Switch, Proceedings of the IRE, M. A. Armistead et a1., volume 44, page 1875; December 1956. These articles describe the various types of microwave switches presently'known for controlling low power microwave energy in waveguides. The ordinary type of waveguide switch described consists in placing an N-type point contact germanium diode across the center of the broad wall of the waveguide and impressing a reverse or forward bias voltage thereby causing the diode to reflect or transmit microwave energy. The crystal represents one of two different impedances which are determined by the amplitude and the polarity of the applied direct current bias. In the passing or on condition, a positive bias is normally applied Where the crystal is centered on the waveguide axis which results in the non-linear resistance being small compared to the barrier capacitance thereby shunting it. The result is a series RL circuit shunted across the waveguide. In the stop or off condition, a back bias is applied to the normal waveguide switch which causes the non-linear resistance to attain high values and thus be shunted by the barrier capacitance.

The ratio of the microwave power passed by the diode in the reflecting state, to the incident microwave power, defines the isolation in db. The same ratio in the transmitting state, defines the insertion loss. A representative insertion for this type of device is 1.0 db.

Other types of switches disclosed in the cited references include the reversed switch which consists in placing a germanium diode in the center of the narrow wall of a waveguide in comparison to the placement of the diode in the center of the broad wall in the switch previously described.

Also disclosed by Garver et al. is a hybrid T semiconductor switch designed for a particular frequency. In this 'ice type of device, silicon diodes are placed in two appropriate arms of the hybrid T and are backed by fixed waveguide shorts. By applying direct current or pulse voltages to two germanium diodes, a reversed type of switch is obtained. Insertion losses of 0.7 db and isolations of 50 db are obtained at 1 milliwatt incident microwave peak power.

Accordingly, an object of the present invention is to provide an improvement in microwave switches.

Another object of the invention is to provide a microwave switch which has very low insertion loss.

Another object of the present invention is to provide a microwave switch wherein the peak isolation is tunable over the greater part of a waveguide frequency range.

These and other objects will become clearly apparent after a study of the following specification, when read in connection with the accompanying drawings, in which:

FIGURE 1 illustrates a first embodiment of the present invention;

FIG. 2 is a schematic diagram of the embodiment shown in FIG. 1;

FIG. 3 illustrates a second embodiment of the present invention in which a T type junction is utilized; and,

FIG. 4 is a graph showing the relationship of the insertion loss of the present invention with relationship to diode placement for two widely diflerent frequencies.

In the present invention, a new and novel semiconductor switch configuration has been found to yield an insertion loss far less than that which has been able to be achieved with present devices. Rather than mounting the diode in the center of the main transmission line, the device embodying the present invention mounts a single diode in the side arm of a three port microwave junction adjacent to or coincident with the port, centered in the plane of the inner wall of the main arms.

A waveguide short in the side arm can permit either complete transmission or complete reflection of power between the other two arms depending on its phase or position relative to its respective port. For example with reference to FIGS. 1 and 2, a flat faced slidingshort 15 will completely block the arm of symmetry 14 of an H- plane Y T It) if it is located at the port 30 of the arm, flush with the guide wall of arms 12 and 16; the main transmission lines 12 and 16 is then effectively a length of continuous waveguide which gives nearly perfect transmission.- As this short is moved away from the port 30, a distance B, a point of maximum reflection is reached approximately a quarter wave length away from the port and any odd number multiples thereof. A crystal diode, not shown, located in the side arm 14 in front of shorting plate 15 can be made to cancel the eliectt of the shorting plate itself and appear as though the short were located at the junction of the arm flush with the main guide wall.

In the first embodiment of the present invention as illustrated in FIG. 1 and FIG. 2, a detector type crystal mount 18 is located at a small distance A away from the terminal plane T at port 30. This distance A can vary from zero to 1.0 centimeter. The terminal plane T is an arbitrary reference plane coincident with the inner wall of the main arms 16 and 12 which simplifies the equivalent circuit of the microwave structure. The terminal plane for arm 16 is shown as T and the terminal plane for arm 12 is shown as T The choice of terminal planes, as shown, was dictated by N. Marcuvitz, Wave Guide Handbook, MIT Radlab Series, volume 10, McGraw Hill Book Com pany, Inc., New York, New York, page 363. Likewise, the terminal plane H, plane T of the T junction is illustrated by Marcuvitz, supra page 355. By choice of the terminal plane, the equivalent reactances representing the two embodiments shown in FIGS. 1 and 3 may be evaluated. It then becomes possible, through the application of network theory to calculate the reactive loads that must be placed on the shunt and series arm, with a matched resistive load to convert the equivalent impedance of the network into a pure reactance. This calculation, however, is extremely tedious and it is convenient to determine the most satisfactory position for the diode and the short circuit experimentally. This is also true because of the imperfect conductivity of the waveguide walls which introduces an error in computations.

As already noted, a crystal mount 18 is located in one arm of the H-plane T in FIG. 1 in one arm 14. A shorting plate is located within arm 14 and is adjustable by means of the adjusting means 11. The shorting plate 15 is adjusted such that it is located a distance B from the waveguide port 30. The crystal mount 18 is located at point C which is centered in the waveguide a distance A less than 1.0 centimeter away from the port however, it can also be centered at the mid-point of the port 30 where the distance A is equal to zero.

The subject invention achieves a new and unusual result of the unusually small insertion loss in comparison to prior art devices. Insertion loss of approximately 0.10 db is obtained. The closest prior art device was not able to achieve the results of the present invention due to the fact that the hybrid T switch disclosed by Garver et al. made it a physical impossibility to place a crystal mount within the region shown as the distance A in FIG. 2 which is less than 1.0 centimeter and more particularly approximately 0.4 centimeter as shown in FIG. 4. Further, referring to FIG. 4, a hybrid T would have to locate the H-plane arms crystal in the region greater than 2.0 centimeters since it cannot physically get close to the junction. Also a tunable short would be ineffective in the 2.0 centimeter region because tuning for lower frequency than 9.8 kmc. would run into the tremendous peak of insertion loss pictured in the graph. Here the peak moves out as the frequency decreases.

Although the distance A from the waveguide port 30 is substantially constant, it is not a rigidly fixed distance but is one which is slightly variable with respectto the type of'diode being utilized. This is due to the physical characteristics of the diode. However, with a known diode the optimum position may be determined experimentally. The diode being utilized is placed .within the crystal diode mount 18 and secured by the crystal holder 13 which is an integral part of the mount itself. Also associated with the mount is an electrical connector 19,

preferably being a BNC connector for the application of a bias voltage to the particular diode used.

The second embodiment as shown in FIG. 3 is similar to the first embodiment of FIG. 1 in that a crystal diode mount 28 comprising a crystal holder 21 and electrical connector 27 is located in the side arm 24 of an H-plane T junction 20. An adjustable shorting plate 26 is located in side arm 24, having adjusting means 29 associated therewith.

In operation, the first embodiment shown in FIG. 1 has microwave energy fed into junction 10 by means of the symmetrical arm '16 and assuming the switch being in the on condition, the microwave energy exits the junction by means of the microwave arm 12. In the on position, the bias voltage applied through connector 19 into the diode mount 18 reverse biases the diode thereby providing a substantial short circuit across the shorting plate 15 located at a distance B which is substantially a quarter of a wave length away from the port 30 and the terminal plane T1 as shown in FIG. 2. The reversed bias diode presents a closed path across the junction .port

thereby presenting a virtual continuous waveguide for the passage of microwave energy from arm 16 through arm 12. In the .oif position, a forward bias voltage is applied to the connector 19, preferably a BNC connector, such that the crystal diode no longer affects the presence of the shorting plate but in a sense electrically disappears,

allowing said shorting plate 15 to prevent the flow of microwave energy through arms 16 and 12.

7 It should be noted that the polarity of the bias is exactly opposite to that required for the normal waveguide switch wherein the diode is mounted in the center of the broad wall in the main transmission line; In addition, as a basis of'comparison, an identical IN419 diode was used in both types of switches with the following results: in the normal switch 28 db isolation was obtained with a minimum insertion loss of 0.7 db and 1.3 VSWR; in the switch comprising the present invention 14 db peak isolation was obtained with less than 0.1 db insertion loss and 1.08 VS'WR.

Similarly, the waveguide switch utilizing the H-plane T junction, as shown in FIG. 3, allows energy to pass through the waveguide members 22 and 25 without any attenuation excepting the very low insertion loss (0.1 db) when the bias voltage applied to the BNC connector 27 of crystal diode mount 28 is of proper polarity to bias the diode contained therein in a reverse direction. The application of a positive bias allows the shouting plate 26 disposed within arm 24 to cause high reflection, blocking the flow of microwave energy through the waveguide arm 22. I

FIG. 4 is a graphical representation of the insertion loss of the diode with respect to its location away from the junction at two widely separated frequencies, namely 7.6 kilomegacycles and 9.8, kilomegacycles. It should be pointed out that for location of the diode and crystal mount less than 1.0 centimeter away the junction or port, the insertionloss is relatively insensitive to frequency, thereby providing a switch which is tunable. It should also be pointed out that in this particular embodiment showing the illustrative graph, the optimum dimension appears at approximately 0.4 cm. from the waveguide port or terminal plane wherein an insertion loss in the order of 0.1 db is indicated. As the distance away from the port'is increased past 1.0 centimeter, the insertion loss rises very'rapidly thus giving rise to the high insertion losses noted by the prior art devices. Since the insertion loss in the present invention is greatly reduced, the possibilities of cascading several of these switches to obtain high isolation make it extremely useful. Further, by proper biasing methods, the present device may be used asanRF modulator.

Whereas I have shown and described my invention with respect to an embodiment thereof which gives satisfactory results, it should be understoodthat changes may be made and equivalents substituted without departiing from the spirit and scope of the invention.

a I claim as my invention:

1. Apparatus for controlling microwave power comprising: a three port microwave junction having three waveguide members; a tunable shorting plate disposed within a first said waveguide member at a substantially rm/4 distance from a terminal: plane as defined by the intersection of said first waveguide member with the second and third waveguide members, where n equals an odd whole number and A represents'the wavelength of said microwave power; one semiconductor diodemeans 7 located within said first waveguide member between said shorting plate and said first port coincident with said terminal plane, said diode means being electrically opera- 5 tive in a first bias condition for permitting substantially complete transmission of microwave energy between the remainder of said waveguide members but electrically operative in a second bias condition for permitting said shorting plate to prevent the flow of microwave energy three port microwave junction having three microwave waveguide branch members; an adjustable shorting plate disposed within a first of said three waveguide members,

'saidshorting plate being located a distance substantially M4 away from a terminal plane defined by the intersection of said first waveguide member with the inner wall of the second and third waveguide members; a single semiconductor diode means, said diode means being located from zero to substantially 1.0 centimeter from said terminal plane; bias means electrically connected to said diode means, said diode means being operative by means of a reverse bias for permitting substantially complete transmission of microwave energy between the remaining said waveguide members but electrically operative in a forward bias condition for permitting said shorting plate to prevent the flow of microwave energy between said remaining waveguide members.

3. Microwave apparatus comprising: a plurality of waveguide members forming an H-plane microwave junction; a shorting plate supported within a first waveguide member substantially one quarter wavelength distance away from the respective port of said first member, a semiconductor diode means located within said first waveguide member from zero to a preselected distance away from the midpoint of said port of said first waveguide member, said diode means being electrically operative in a reversed bias condition for providing a short circuit across said shorting plate thereby permitting substantially complete transmission of microwave energy between the remainder of said waveguide members, but electrically operative in a forward bias condition for permitting said shorting plate to reflect microwave energy passing between the remainder of said waveguide means.

4-. Apparatus for controlling microwave power: an I-I- plane Y microwave junction having three symmetrical waveguide members; a shorting plate disposed within a first waveguide member at a point substantially one quarter wavelength away from a terminal plane defined by the intersection of said first waveguide member with the inner walls of the second and third waveguide members; a single semiconductor diode means, said diode means being located within said first waveguide member a distance from zero to substantially 1.0 centimeter away from the center line of said terminal plane, said diode being reversed biased in a first condition for permitting substantially complete transmission of microwave energy between the remainder of said waveguide members by providing a short circuit across said shorting plate, but electrically operative in a forward biased condition for permitting said shorting plate to substantially prevent the flow of microwave energy between said remaining waveguide members; and bias means electrically connected to said diode means for providing a bias potential.

5. Microwave apparatus comprising: an H-plane T having three waveguide members and three respective microwave ports formed by their junction; a microwave energy source electrically connected to a first waveguide member of said H-plne T; a shorting plate disposed within a second waveguide member of said T an odd number of quarter Wavelengths away from the port of said second waveguide member; a single semiconductor diode disposed within said second waveguide member a distance A between the said shorting plate and the port of said second waveguide member, said distance A having from 0 to substantially 1.0 centimeter, said diode being electrically operative in a first bias condition for permitting substantially complete transmission of microwave energy between said first waveguide member and a third waveguide member but electrically operative in a second bias condition for permitting said shorting plate to attenuate the flow of microwave energy between said first and third microwave waveguide members, and bias means electrically connected to said diode means for providing said first and second bias conditions.

References Cited by the Examiner UNITED STATES PATENTS 3,038,086 6/62 Sterzer 307-88.5 3,069,629 12/62 Wolff 328-92 OTHER REFERENCES Graver et al.: Microwave Semiconductor Switching Techniques, IRE Transactions on Microwave Theory and Techniques, volume MTT-6, October 1958, pages 378 to 383.

HERMAN KARL SAALBACH, Primary Examiner. 

1. APPARATUS FOR CONTROLLING MICROWAVE POWER COMPRISING: A THREE PORT MICROWAVE JUNCTION HAVING THREE WAVEGUIDE MEMBERS; A TUNABLE SHORTING PLATE DISPOSED WITHIN A FIRST SAID WAVEGUIDE MEMBER AT A SUBSTANTIALLY N$/4 DISTANCE FROM A TERMINAL PLANE AS DEFINED BY THE INTERSECTION OF SAID FIRST WAVEGUIDE MEMBER WITH THE SECOND AND THIRD WAVEGUIDE MEMBERS, WHERE N EQUALS AN ODD WHOLE NUMBER AND $ REPRESENTS THE WAVELENGTH OF SAID MICROWAVE POWER; ONE SEMICONDUCTOR DIODE MEANS LOCATED WITHIN SAID FIRST WAVEGUIDE MEMBER BETWEEN SAID SHORTING PLATE AND SAID FIRST POR T COINCIDENT WITH SAID TERMINAL PLANE, SAID DIODE MEANS BEING ELECTRICALLY OPERATIVE IN FIRST BIAS CONDITION FOR PERMITTING SUBSTANTIALLY COMPLETE TRANSMISSION OF MICROWAVE ENERGY BETWEEN THE REMAINDER OF SAID WAVEGUIDE MEMBERS BUT ELECTRICALLY OPERATIVE IN A SECOND BIAS CONDITION FOR PERMITTING SAID SHORTING PLATE TO PREVENT THE FLOW OF MICROWAVE ENERGY BETWEEN SAID REMAINING WAVEGUIDE MEMBERS; AND BIAS MEANS FOR APPLYING A BIAS VOLTAGE TO SAID DIODE MEANS. 